A typical carpet is composed of four distinct materials: 1) the face fiber or pile, often made of nylon, polyethylene terephthalate, or polyolefin; 2) the backing, often made of polypropylene or natural fibers like jute; 3) the binder or adhesive, often a styrene butadiene rubber (SBR) latex; and 4) the inorganic filler, often calcium carbonate or limestone. Thousands of tons of post-consumer and post-industrial carpet are sent to landfills each year. These carpets are difficult to recycle, being mixtures of various plastics, binders, inorganic fillers, and fibers. Many high-end uses of recycled carpet require separating the different polymers, removing dirt and other contaminants, and removing the binders and other fillers. By design, carpeting is difficult to deconstruct. It is built to survive decades of wear without having the face fiber pull free from the backing material, and without the binder failing.
The most valuable portion of the carpet is typically the “face fiber” material, also sometimes described as “pile,” which is often a polyester or a nylon, and typically makes up from about 35 to about 65 wt. % of the carpet. Carpet to be recycled is often sorted based on its face fiber composition, with nylon carpet currently being more valuable due to the higher demand for recycled nylon.
One conventional method for recovering the face fiber material typically involves shearing, a method for removing face fiber material analogous to shearing a sheep to remove its fleece. In such methods, the balance is typically discarded. An advantage of this method is that most of the non-face fiber portion of the carpet is separated from the face fiber. A disadvantage of the method is that shearing is labor intensive. Pieces of carpet must be unrolled, cut into appropriately-sized pieces, and manually fed one-by-one into a shearing unit. The carpet must be fed into the shearer in the proper orientation, with the face fiber oriented toward the shearing blades, making the sheared fiber susceptible to contamination with the backing material. A further disadvantage of this method is that the yield of face fiber is low—typically only 25% to 50% of the face fiber is recovered.
In shearing, the cutting depth must be carefully adjusted to maximize face fiber recovery while minimizing cutting into the backing material. Unfortunately, carpet thicknesses vary. Deep cuts risk contaminating the sheared fiber, while shallow cuts result in yield losses. Cutter wear in such applications is significant and costly.
Another conventional method of carpet recycling is whole carpet shredding. The entire carpet is simply shredded into fibers, and a portion of the latex and inorganic filler are removed as sand or dust. However, this method has the disadvantage of leaving the backing polypropylene fibers still intermixed with the face fibers. Furthermore, the bottom end of each face fiber retains a significant portion of the latex and inorganic filler, making this face fiber unsuitable for uses that require a more purified recycled face fiber.
U.S. Pat. No. 5,889,142 discloses a process for selectively separating polyamides from multi-component waste materials that includes the steps of subjecting the multi-component mix to a specific mixture of caprolactam and water at a preselected temperature range below the degradation temperature of the polymer to be recovered, separating the formed polyamide solution, and recovering the desired polyamide.
U.S. Pat. No. 7,067,613 discloses, in the recycling of Nylon 6 and Nylon 6,6 polyamides from post-consumer or post-industrial waste, a process to separate the polyamides from commingled polyolefin waste components, particularly polypropylene, by admixing the waste with an ester solvent composition and heating the admixture to a temperature above the melting temperature of the contained polyolefins to form an ester solvent composition further containing dissolved polyamide polymer and a separate immiscible liquid polyolefin phase.
U.S. Pat. No. 6,752,336 discloses a method of recovering carpet materials by reducing carpet into size-reduced fibers, slurrying the size-reduced fibers in a liquid medium, and then selectively separating the size-reduced fibers in a centrifuge. The method is said to be particularly appropriate for recovering nylon or polyester face fibers from post-industrial, pre-consumer carpet waste.
U.S. Pat. No. 6,498,250 discloses a process for nylon depolymerization in which a multi-component material, comprising nylon and one or more non-nylon components, is fed to a depolymerization zone in which depolymerization of at least part of the nylon is effected, resulting in a product stream and a residue, the product stream containing monomers of the nylon, and the residue containing non-nylon components, in which the nylon content in the residue is measured and used to control the depolymerization process.
U.S. Pat. No. 7,784,719 discloses methods of recovering primary construction materials from whole carpet that are said to be particularly appropriate for recovering nylon or polyester face fibers from post-industrial, post-consumer carpet waste. The methods include reducing the whole carpet into fragmented carpet materials (i.e., pile, backing, and binder), further reducing the fragmented carpet materials into size-reduced fibers and binder, slurrying the size-reduced fibers and binder in an aqueous liquid medium (e.g., water), and then separating the size-reduced fibers and binder in a centrifuge.
U.S. Pat. Publn. No. 2011/0040027 discloses a method of recycling carpet components that comprises converting post-consumer carpet that includes a latex backing into a free-flowing powder that is said to be suitable for incorporation into one or more products as a recycled product component. Various processes of converting post-consumer carpet comprising a latex backing are disclosed, including a process in which a portion of face fibers may be harvested from used, post-consumer carpet, or the carpet may simply be shredded to form a first mixture, after which a portion of carpet fibers are separated from and removed from the first mixture to form a second mixture, which may subsequently be exposed to a relatively high level of heat to thermally degrade and/or partially volatilize polymeric material present in the second mixture. This mixture may then be incorporated into new products, either alone or admixed with a solid inorganic particulate material.
Notwithstanding the carpet recycling methods just described, there remains a need in the art for improved processes for recycling carpet, especially those containing face fibers such as nylons and polyesters.